There is a present need to decrease carbon dioxide (CO2) emissions from industrial facilities. Over the years, a number of electrochemical processes have been suggested for the conversion of CO2 into useful products. Processes for CO2 conversion and the catalysts for them are discussed in U.S. Pat. Nos. 3,959,094, 4,240,882, 4,523,981, 4,545,872, 4,595,465, 4,608,132, 4,608,133, 4,609,440, 4,609,441, 4,609,451, 4,620,906, 4,668,349, 4,673,473, 4,711,708, 4,756,807, 4,818,353, 5,064,733, 5,284,563, 5,382,332, 5,457,079, 5,709,789, 5,928,806, 5,952,540, 6,024,855, 6,660,680, 6,987,134 (the '134 patent), U.S. Pat. Nos. 7,157,404, 7,378,561, 7,479,570, U.S. patent application 20080223727 (the '727 application) and papers reviewed by Hori (Modern Aspects of Electrochemistry, 42, 89-189, 2008) (“the Hori Review”), Gattrell, et al. (Journal of Electroanalytical Chemistry, 594, 1-19, 2006) (“the Gattrell review”), DuBois (Encyclopedia of Electrochemistry, 7a, 202-225, 2006) (“the DuBois review”).
Generally, an electrochemical cell contains an anode 50, a cathode 51 and an electrolyte 53 as illustrated in FIG. 1. Catalysts are placed on the anode, and/or the cathode, and/or in the electrolyte to promote desired chemical reactions. During operation, reactants or a solution containing reactants is fed into the cell. Then a voltage is applied between the anode and the cathode, to promote an electrochemical reaction.
When an electrochemical cell is used as a CO2 conversion system, a reactant comprising CO2, carbonate or bicarbonate is fed into the cell. A voltage is applied to the cell, and the CO2 reacts to form new chemical compounds. Examples of cathode reactions in the Hori Review include:CO2+2e−+2H+→CO+H2O2CO2+2e−→CO+CO32−CO2+H2O+2e−→CO+2OH−CO2+2H2O+4e−→HCO−+3OH−CO2+2H2O+2e−→H2CO+2OH−CO2+H2O+2e−→(HCO2)−+OH−CO2+2H2O+2e−→H2CO2+2OH−CO2+5H2O+6e−→CH3OH+6OH−CO2+6H2O+8e−→CH4+8OH−2CO2+8H2O+12e−→C2H4+12OH−2CO2+9H2O+12e−→CH3CH2OH+12OH−2CO2+6H2O+8e−→CH3COOH+8OH−2CO2+5H2O+8e−→CH3COO−+7OH−2CO2+10H2O+14e−→C2H6+14OH−CO2+2H++2e−→CO+H2O, acetic acid, oxalic acid, oxylateCO2+4H++4e−→CH4+O2 where e− is an electron. The examples given above are merely illustrative and are not meant to be an exhaustive list of all possible cathode reactions.
Examples of reactions on the anode mentioned in the Hori Review include:2O2−→O2+4e−2CO32−→O2+2CO2+4e−4OH−→O2+2H2O+4e−2H2O→O2+4H++4e−The examples given above are merely illustrative and are not meant to be an exhaustive list of all possible anode reactions.
In the previous literature, catalysts comprising one or more of V, Cr, Mn, Fe, Co, Ni, Cu, Sn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Hg, Al, Si, In, Tl, Pb, Bi, Sb, Te, U, Sm, Tb, La, Ce, and Nd have all shown activity for CO2 conversion. Reviews include Hori (Modern Aspects of Electrochemistry, 42, 89-189, 2008) (“the Hori Review”), Gattrell, et al. (Journal of Electroanalytical Chemistry, 594, 1-19, 2006) (“the Gattrell review”), DuBois (Encyclopedia of Electrochemistry, 7a, 202-225, 2006) (“the DuBois review”), and the papers Li, et al. (Journal of Applied Electrochemistry, 36, 1105-1115, 2006, Li, et al. (Journal of Applied Electrochemistry, 37, 1107-1117, 2007, and Oloman, et al. (ChemSusChem, 1, 385-391, 2008) (“the Li and Oloman papers”), and references therein.
The results in the Hori Review show that the conversion of CO2 is only mildly affected by solvent unless the solvent also acts as a reactant. Water can act like a reactant, so reactions in water are different than reactions in non-aqueous solutions. But the reactions are the same in most non-aqueous solvents, and importantly, the overpotentials are almost the same in water and in the non-aqueous solvents.
The catalysts have been in the form of either bulk materials, supported particles, collections of particles, small metal ions or organometallics. Still, according to Bell (A. Bell. Ed, Basic Research Needs, Catalysis For Energy, U.S. Department of Energy Report PNNL17712, 2008) (“the Bell Report”), “The major obstacle preventing efficient conversion of carbon dioxide into energy-bearing products is the lack of catalyst” with sufficient activity at low overpotentials and high electron conversion efficiencies.
The overpotential is associated with lost energy of the process, so the overpotential should be as low as possible. Yet, according to The Bell Report “Electron conversion efficiencies of greater than 50 percent can be obtained, but at the expense of very high overpotentials”. This limitation needs to be overcome before practical processes can be obtained.
A second disadvantage of many of the catalysts is that they also have low electron conversion efficiency. Catalyst systems are considered practical where electron conversion is over 50%.
U.S. patent application Ser. No. 12/830,338 (published as US2011/0237830) and Ser. No. 13/174,365 (published as US2012/0308903), and International Application No. PCT/US2011/030098 (published as WO2011/120021) and PCT/US2011/042809 (published as WO2012/006240) disclose that a catalyst mixture containing an active metal and a Helper Catalyst could catalyze CO2 conversions with low overpotential and high electron conversion efficiency. However, the catalysts disclosed in these patent applications showed a lower activity than was desired.
The patent documents identified above consider applications for CO2 conversion, but the present electrocatalysts overcome limitations of other systems. For example, some commercial CO2 sensors use an electrochemical reaction to detect the presence of CO2. At present, these sensors require over 1-5 watts of power, which is too high for portable sensing applications.